Polarizing a Hydrophobic Cavity for the Efficient Binding of Organic Guests: The Case of Calix[6]tren, a Highly Efficient and Versatile Receptor for Neutral or Cationic Species

The host−guest properties of calix[6]tren 1 have been evaluated. The receptor is based on a
calix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents a
concave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavity
and opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules in
chloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H+, behaved as a remarkable receptor for small organic molecules. The complexation
is driven by a strong charge−dipole interaction and hydrogen bonds between the polar guest and the
tetracationic cap of the calixarene. Finally, coordination of Zn2+ to the tren core led to the asymmetrization
of calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizes
the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. The
resulting combination of charge−dipole, hydrogen bonding, CH−π, and van der Waals interactions highly
stabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of the
polarization, offering either a basic (1), a highly charged and acidic (1.4H+), or a coordination (1.Zn2+) site.
As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simple
addition of protons, bases, or metal ions.